Re: CFQ I/O starvation problem triggered by RHEL6.0 KVM guests

[Vivek Goyal <vgoyal@redhat.com>]

On Fri, Sep 09, 2011 at 06:00:28PM +0900, Takuya Yoshikawa wrote:

[..]
> 
> > 
> > - Even if there are close cooperators, these queues are merged and they
> >   are treated as single queue from slice point of view. So cooperating
> >   queues should be merged and get a single slice instead of starving
> >   other queues in the system.
> 
> I understand that close cooperators' queues should be merged, but in our test
> case, when the 64KB request was issued from one aio thread, the other thread's
> queue was empty; because these queues are for the same stream, next request
> could not come until current request got finished.
> 
>   But this is complicated because it depends on the qemu block layer aio.
> 
> I am not sure if cfq would try to merge the queues in such cases.

[CCing Jeff Moyer ]

I think even if these queues are alternating, it should have been merged
(If we considered them close cooperator).

So in select queue we have.

        new_cfqq = cfq_close_cooperator(cfqd, cfqq);
        if (new_cfqq) {
                if (!cfqq->new_cfqq)
                        cfq_setup_merge(cfqq, new_cfqq);
                goto expire;
        }

So if we selected a new queue because it is a close cooperator, we should
have called setup_merge() and next time when the IO happens, one of the
queue should merge into another queue.

cfq_set_request() {
                if (cfqq->new_cfqq)
                        cfqq = cfq_merge_cfqqs(cfqd, cic, cfqq);
}

If merging is not happening and still we somehow continue to pick 
close_cooperator() as the new queue and starve other queues in the system,
then there is a bug.

I think try to reproduce this with fio with upstream kenrels and put
some more tracepoints and see what's happening.

Thanks
Vivek